1. Field of the Invention
The present invention relates to an antenna device including a surface-mounted element, and in particular to an antenna device which may be built into a radio apparatus.
2. Description of the Related Art
A surface-mounted antenna device is known which is formed by an antenna element mounted on a surface of a dielectric material base. The surface-mounted antenna device may be directly mounted on a housing or a printed circuit board of a small-sized radio apparatus such as a mobile phone, and may be used as a built-in antenna. The surface-mounted antenna device may be called a molded inter-connect device (MID) antenna due to a characteristic shape.
In some cases, an attempt is made to downsize the surface-mounted antenna device where relative permittivity of a dielectric material base of the antenna device may produce a wavelength shortening effect. Meanwhile, the surface-mounted antenna device is required to cover a broad frequency range, which is known to be hardly compatible with downsizing in general. Attempts have been made to cope with both of the above requirements which are generally considered to conflict with each other, as disclosed in Japanese Patent Publication of Unexamined Applications (Kokai), No. 2003-198239.
More specifically, a surface-mounted antenna device disclosed in JP 2003-198239 has a radiation electrode (antenna element) arranged on an upper face of a dielectric material base which is bridge-like shaped by removing a portion of a lower face to arrange a concave portion on the lower face. It is described in JP 2003-198239 that the bridge-like shape may be aligned in a way that balance between radiation efficiency and frequency range characteristics is considered. It is also described in JP 2003-198239 that the surface-mounted antenna device may be modified to have additional piece of dielectric material of relative permittivity different from relative permittivity of the dielectric material base loaded on the concave portion of the lower face.
For being loaded on the concave portion of the lower face of the dielectric material base, the additional dielectric material shall be selected in terms of mechanical characteristics, e.g., requiring adhesiveness so as to be applied to structure of the antenna device of JP 2003-198239. Besides, it is difficult for the antenna device of JP 2003-198239 to be made multiple-frequency resonant by having plural pieces of dielectric material of relative permittivity different one another loaded.